While the first 4-6-2 locomotives still had a narrow firebox and did not bring much advantage over the 4-6-0, the later, classic Pacifics were characterized by a wide firebox above the trailing axle. This allowed a sufficiently large grate area to maintain high speeds with heavy trains, as had been the case with the Atlantic-type express locomotives a few years earlier. While the Pacific was used almost exclusively as express locomotive in Europe, there were also many engines in North America with slightly smaller drivers, which were primarily intended for the heaviest, more medium-speed passenger trains.

The first model of this type were the 21 Missouri Pacific locomotives, which were built by ALCO-Brooks in 1902 and formed the basis for the name “Pacific”. They were lighter than other Pacifics that were built in the following years and had a driver diameter of only 69 inches. Together with the abbreviation "P" for “Pacific”, this led to the class designation P-69.

Since the locomotives were partly built directly for the Missouri Pacific and partly for their subsidiary St. Louis, Iron Mountain and Southern Railway, they were given separate number ranges between 1115 and 1123 and between 1616 and 1623. The Stephenson valve gear was later replaced on 20 engines by a Baker valve gear and on one engine by the Walschaerts valve gear. In 1916 they were superheated, where the cylinder diameter was increased by one inch. The last examples served in this form until 1953.